The overall objective of this project is to study the role of tum r antigens in the immune response of the tumor-bearing host to its tumor and to determine whether immunologic manipulations of the immune response of the host can increase the host's resistance against the tumor. There is ample evidence that tumors release antigen from the cell surface and that this antigen forms complexes with the existing antibody in the circulation. Although the respective roles of free antigen, antigen-antibody complex, or free antibody have been demonstrated in vitro, the role of these same materials in the host's immune response in vivo is less clear. Therefore, the specific objectives of the project are first, to isolate tumor antigen-antibody complexes from the tumor-bearing host and determine their effect on tumor growth in recipient animals, and second, to inject labeled tumor cells or tumor antigens into normal, immune, tumor-bearing or immunosuppressed, syngeneic and allogeneic mice to determine the rate of clearance of these tumor cells and tumor antigens from the circulation. The effect of treatment of tumor-bearing hosts with cyclophosphamide or C. parvum on the rate of clearance of tumors will also be investigated. The ultimate goal of these experiments is to look for new and rational models of immunotherapy based on a better understanding of the effect of tumor growth on the immune response of the host.